Heat transfer assembly

ABSTRACT

A heat transfer assembly having a heat spreading member sandwiched between a heat source and a heat sink is disclosed. The heat sink, the heat spreading member, and the heat source are pressed against the bottom of a substrate support plate by a bias member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/440,365, filed May 16, 2003, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to semiconductor substrateprocessing systems. More specifically, the invention relates to anapparatus for supporting a substrate in a semiconductor substrateprocessing system.

2. Description of the Related Art

Accurate reproducibility of substrate processing is an important factorwhen increasing productivity for integrated circuit fabricationprocesses. Precise control of various process parameters is required forachieving consistent results across a substrate, as well as results thatare reproducible from substrate to substrate. More particularly,uniformity of the substrate temperature during processing is onerequirement for achieving accurate reproducibility. During substrateprocessing, changes in the temperature and temperature gradients acrossthe substrate are detrimental to material deposition, etch rate, stepcoverage, feature taper angles, and the like.

Generally, during processing, the substrate is disposed on a substratesupport (e.g., electrostatic chuck, susceptor, and the like) that isthermally coupled to a heat source, such as an embedded heater, e.g., aresistive heater and the like. Additionally, in some applications, heatis also produced by the process itself (e.g., plasma process). Toenhance the processing and minimize undesirable yield losses, it isessential to control the temperature as well as the temperatureuniformity of the substrate.

Therefore, there is a need in the art for a substrate support havingmeans to control the temperature as well as the temperature uniformityof the substrate.

SUMMARY OF THE INVENTION

The disadvantages associated with the prior art are overcome by animproved substrate support for a semiconductor substrate processingsystem. The substrate support comprises a heat transfer assembly havinga heat spreader member that is sandwiched between a heat source and aheat sink. The heat sink, heat spreader member, and heat source arepressed against the bottom of a substrate support plate by a biasmember.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a schematic diagram of an exemplary processing reactorcomprising a substrate support in accordance with one embodiment of thepresent invention;

FIG. 2 is a schematic, cross-sectional view of a heat transfer assemblyof the substrate support of FIG. 1 in accordance with one embodiment ofthe present invention; and

FIG. 3 is a schematic, top plan view of the heat transfer assembly ofFIG. 2.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

It is to be noted, however, that the appended drawings illustrate onlyexemplary embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

DETAILED DESCRIPTION

The present invention is a heat transfer assembly for controlling thetemperature and temperature uniformity of a substrate support in asubstrate processing system. The substrate support is generally used tosupport a substrate (e.g., silicon (Si) wafer) in a process chamber ofthe substrate processing system, such as a plasma etching reactor, areactive ion etching (RIE) reactor, a chemical vapor deposition (CVD)reactor, a plasma enhanced CVD (PECVD) reactor, a physical vapordeposition (PVD) reactor, an electron cyclotron resonance (ECR) reactor,a rapid thermal processing (RTP) reactor, an ion implantation system,and the like. The invention is useful in applications that require asubstrate to be supported in a chamber while the temperature of thesubstrate is required to be substantially uniform.

FIG. 1 depicts a schematic diagram of an exemplary Decoupled PlasmaSource (DPS II) etch reactor 100 that may be used to practice theinvention. The DPS II reactor is commercially available from AppliedMaterials, Inc. of Santa Clara, Calif. The particular embodiment of thereactor 100 shown herein is provided for illustrative purposes andshould not be used to limit the scope of the invention. For example, theinvention can be used in apparatus other than a system for processingsubstrates, whether fabricated of semiconductor materials or othermaterials.

The reactor 100 comprises a process chamber 110 and a controller 140.

The process chamber 110 generally comprises a conductive body (wall) 130having a substantially flat dielectric ceiling 120 and encompassing asubstrate support 116. The process chamber 110 may have other types ofceilings, e.g., a dome-shaped ceiling. The wall 130 typically is coupledto an electrical ground terminal 134.

Above the ceiling 120 is disposed an antenna comprising at least oneinductive coil element 112 (two co-axial elements 112 are shown). Theinductive coil element 112 is coupled, through a first matching network119, to a plasma power source 118. The plasma power source 118 generallyis capable of producing up to 5000 W at a tunable frequency in a rangefrom about 50 kHz to 13.6 MHz. The matching network 119 and the plasmapower source 118 are controlled by the controller 140.

The support pedestal 116 is coupled, through a second matching network124, to a biasing power source 122. The biasing power source 122generally is a source of up to 2000 W of continuous or pulsed power at afrequency of approximately 13.6 MHz. In other embodiments, the biasingpower source 122 may be a DC or pulsed DC power source. The biasingpower source 122 and the matching network 124 are controlled by thecontroller 140.

During processing, a substrate 114 is placed on the support pedestal 116and thereafter process gases are supplied from a gas panel 138 throughat least one entry port 126 to form a gaseous mixture 150 in the processchamber 110. Operation of the gas panel 138 is controlled by thecontroller 140. The gaseous mixture 150 is ignited to a plasma 155 inthe process chamber 110 by applying power from the plasma source 118 tothe at least one inductive coil element 112, while the substrate 114 maybe also biased by applying power from the biasing source 122 to thesubstrate support 116.

The lift mechanism 162, as controlled by the controller 140, is used toraise the substrate 114 off the substrate support 116 or to lower thesubstrate onto the substrate support. Generally, the lift mechanism 162comprises an actuator that engages a lift plate (both are not shown)coupled to a plurality of lift pins 172 (one lift pin is illustrativelyshown in FIG. 1). The lift pins 172 travel through respective guideholes 188. Illustratively, the guide holes 188 are defined by an innerpassage of tubes 206 that are supported by bushings 208 (discussed inreference to FIG. 2).

In one embodiment, the guide holes 188 are equidistantly distributedalong a circle 310 (shown in phantom in FIG. 3) that is concentric withthe substrate support 116. Such lift mechanism is disclosed in commonlyassigned U.S. patent application Ser. No. 10/241,005, filed Sep. 10,2002 (Attorney docket number 7262), which is incorporated herein byreference.

Gas pressure within the interior of the chamber 110 is controlled by thecontroller 140 using a throttle valve 127 and a vacuum pump 136. Thetemperature of the wall 130 may further be controlled usingliquid-containing conduits (not shown) running through the wall. Theprocess chamber 110 also comprises conventional systems for processcontrol, including, for example, internal process diagnostics, and thelike. Such systems are collectively depicted in FIG. 1 as supportsystems 107.

To facilitate control of the components and substrate processing withinthe chamber 110, the controller 140 may be one of any form ofgeneral-purpose computer processor that can be used in an industrialsetting for controlling various chambers and sub-processors. Thecontroller 140 generally comprises a central processing unit (CPU) 144,a memory 142, and support circuits 146 for the CPU 144.

Those skilled in the art will understand that other forms of processchambers may be used to practice the invention, such as electroncyclotron resonance (ECR) chambers, chemical vapor deposition (CVD)chambers, plasma enhanced CVD (PECVD) chambers, physical vapordeposition (PVD) chambers, rapid thermal processing (RTP) chambers, andany other chamber that may incorporate a substrate support having anembedded heater therein.

In one depicted embodiment, the support pedestal 116 comprises asubstrate support plate 160, a heat source (such as an embedded heater)132, a heat transfer assembly 164, a heat sink (such as a cooling plate)166, at least one bias member 190, and a mounting assembly 106. Inalternative embodiments, the substrate support plate 160 may comprise anelectrostatic chuck (as shown) or another substrate retention mechanism,e.g., a mechanical chuck, a susceptor clamp ring, vacuum chuck, and thelike.

In operation, the substrate 114 generally should be heated to apre-selected temperature (e.g., from about 0 to 500 degrees Celsius).The substrate 114 is heated with minimal non-uniformity across thesubstrate and then maintained at such temperature. The temperature ofthe substrate 114 is controlled by stabilizing the temperature of thesupport pedestal 116 using an embedded heater 132 and a heat transfergas (e.g., helium (He)). The embedded heater 132 is used to heat thesupport pedestal 116 while the heat transfer gas cools down thesubstrate 114. Generally, helium is provided to the underside of thesubstrate 114 from a source 148 through a gas conduit 149 to channelsand grooves (not shown) formed in a top surface 174 of the substratesupport plate 160.

In one embodiment of the substrate support plate 160, an electrostaticchuck comprises at least one clamping electrode 180 that may beconventionally controlled by a chuck power supply 176. The embeddedheater 132 (e.g., resistive electric heater) comprises at least oneheating element 182 and is regulated by a heater power supply 178.

In one embodiment, the embedded heater 132 is a detachable heater thatis thermally coupled to a bottom surface 133 of the substrate supportplate 160. The at least one bias member 190 applies force to the heater132 to press it against the bottom surface 133 of the substrate supportplate 160. In an alternative embodiment, the heater 132 may be embeddedin the substrate support plate (e.g., electrostatic chuck) 160 or bebonded to the bottom surface 133 of the substrate support plate 160.

The substrate support plate 160 and embedded heater 132 are generallyformed from dielectric materials having a high thermal conductivity(e.g., aluminum nitride (AlN) and the like), as well as low coefficientsof thermal expansion. The coefficients of thermal expansion for each ofthe substrate support plate 160 and the heater 132 should be matched.The high thermal conductivity increases thermal coupling between thesubstrate support plate 160 and the heater 132 to facilitate uniformtemperatures for the support surface 174 of the plate 160 and asubstrate 114 thereon. The matching low coefficients of thermalexpansion reduce the expansion/contraction of the substrate supportplate 160 relative to the heater 132 across a broad range oftemperatures (e.g., from about 0 to 500 degrees Celsius).

The heat transfer assembly 164 facilitates a controlled heat sink pathto the cooling plate 166, for heat generated by the embedded heater 132,as well as for heat produced during substrate processing, e.g., plasmaprocessing. By regulating the total and local thermal conductivity ofthe heat transfer assembly 164, temperature uniformity for the substratesupport 116 may be achieved.

The heat transfer assembly 164 is used to selectively optimize over abroad range of temperatures and process parameters the thermalproperties (i.e., temperature uniformity and maximum temperature) of thesubstrate support 116. In one embodiment, the heat transfer assembly 164comprises an electrostatic chuck and embedded heater. The electrostaticchuck and embedded heater may each be of a variety of designconfigurations. Specifically, the heat transfer assembly 164 may be usedto selectively optimize the thermal properties of a substrate support116 having a detachable embedded heater, e.g., resistive electricheater.

The cooling plate 166 is thermally coupled to the heat transfer assembly164 and generally, is formed from a metal, such as aluminum (Al), copper(Cu), stainless steel, and the like.

In the depicted embodiment, the cooling plate 166 comprises a pluralityof recesses 192, e.g., blind holes, grooves, and the like. Each recess192 houses a bias member 190, including at least one cylindrical springand the like. The bias member 190 exerts an expanding elastic force.Such force engages the substrate support plate 160, embedded heater 162,heat transfer assembly 164, and cooling plate 166 against one anotherand facilitates thermal coupling between the components of the substratesupport 116.

The bias members 190 are disposed such that the substrate support plate160, embedded heater 162, heat transfer assembly 164, and cooling plate166 are uniformly compressed against one another to provide thermalcoupling between the components. In one exemplary embodiment, the biasmembers 190 are disposed along at least one circle that is concentricwith the substrate support 116, e.g., around the lift pins 172.Alternatively, the bias members 190 may be similarly disposed inrecesses that are formed in a surface 169 of the base plate 168, or bothin the cooling plate 166 and base plate 168.

The mounting assembly 106 generally comprises a base plate (or ring)168, a collar ring 184, a flange 162, and a plurality of fasteners(e.g., screws, bolts, clamps, and the like) 167. The fasteners 167couple the flange 162, cooling plate 166 and base plate 168 together toprovide mechanical integrity for the substrate support 116. In furtherembodiments (not shown), the support pedestal 116 may also includevarious process-specific improvements, e.g., a purge gas ring, liftbellows, substrate shields, and the like.

In one embodiment, the collar ring 184 is formed from KOVAR (i.e., analloy comprising, by weight, about 54% iron (Fe), 29% nickel (Ni), and17% cobalt (Co)). Further, the collar ring 184 is brazed to thesubstrate support plate 160 and flange 162 to facilitate gas-tightcoupling between the support plate and flange. KOVAR has a lowcoefficient of thermal expansion and a low thermal conductivity and isknown in the art for forming strong brazed bonds with materials, such asceramics (support plate 160) and metals (flange 162). KOVAR iscommercially available from EFI of Los Alamitos, Calif., and othersuppliers.

The mounting assembly 106 encompasses an interior region 186 of thesubstrate support 116. In operation, the interior region 186 generallyis maintained at a gas pressure that is higher than the gas pressure ina reaction volume 141. Such higher gas pressure (e.g., atmosphericpressure) prevents radio-frequency arcing within the support pedestal116 that otherwise is promoted by the biasing power source 122.

FIG. 2 and FIG. 3 are, respectively, schematic, cross-sectional and topplan views of a heat transfer assembly 164 of a substrate support 116 ofthe reactor 100. The cross-sectional view in FIG. 2 is taken along acenterline 3-3 in FIG. 3.

Referring to FIGS. 2 and 3, in one illustrative embodiment, the heattransfer assembly 164 comprises a heat spreader plate 254 that issandwiched between a first contact plate 256 and a second contact plate258. Alternatively, the heat transfer assembly 164 may comprise a singlecomposite sandwich-like member. Additionally, the embedded heater 132may be included in the heat transfer assembly 164.

The first and second contact plates 256, 258 are used to reduce, in acontrolled manner, heat flux from the embedded heater 132 through theheat transfer assembly 164 to the cooling plate 166. The cooling plate166 comprises conduits 210 that facilitate coolant flow to remove heatfrom the cooling plate 166. The contact plates 256, 258 may be formedfrom materials having a low thermal conductivity, e.g., KOVAR, titanium(Ti), and the like. Generally, the contact plates 256, 258 have athickness of about 3 to 12 mm.

The first contact plate 256 has a flat (i.e., smooth) first contactsurface 256A and an embossed second contact surface 256B. Similarly, thesecond contact plate 258 has a smooth first contact surface 258A and anembossed second contact surface 258B. The smooth first contact surfaces256A, 258A engage a bottom surface 202 of the embedded heater 132 and abottom surface 204 of the heat spreader plate 254, respectively.Accordingly, the embossed second contact surface 256B engages a topsurface 203 of the heat spreader plate 254, while the embossed secondcontact surface 256B engages a top surface 205 of the cooling plate 166.

A surface area of the embossed second contact surfaces 256B, 258Bgenerally comprises about 5 to 50% of the surface area of the smoothfirst contact surfaces 256A, 258A, respectively. The second contactsurfaces 256B, 258B may be embossed using conventional machiningtechniques, such as milling, turning, and the like.

Contact plates with a smaller embossed surface area provide acorresponding lower thermal conductivity in the direction that isorthogonal to the smooth contact surfaces. This means that contactplates 256, 258 having a smaller embossed surface area will reduce theheat flux from the embedded heater 132 at a slower rate then contactplates having a larger embossed surface area. In one exemplaryembodiment, the surface area of the embossed contact surfaces 256B, 258Bcomprise about 20% of the surface area of the respective first contactsurfaces 256A, 258A.

Further, a local pattern density for the embossed surfaces 256B, 258B(i.e., surface area in a specific region of contact surface 256B or258B) may be selected such that a contact plate has a pre-determinedlocal thermal conductivity. The pre-determined local thermalconductivity in one region of a contact plate may be higher or lowerthan the thermal conductivity in other regions of the plate. Suchcontact plates may be used to control the flux of heat in specificregions of the heat transfer assembly 164 to improve temperatureuniformity across the substrate support plate 160 as well as thesubstrate 114.

The heat spreader plate 254 reduces temperature non-uniformity caused byfeatures formed in the substrate support 116 (e.g., guide holes 188, gasconduit 149, the embossed surfaces 256B, 258B of the contact plates 256,258, and the like). Generally, the heat spreader plate 254 is formed toa thickness of about 3 to 12 mm of a material having a high thermalconductivity (e.g., aluminum nitride (AlN), copper (Cu), and the like).

The thermal conductivity of the heat transfer assembly 164 may beselectively controlled by choosing the materials and thickness for theheat spreader plate 254 and contact plates 256, 258, as well as apattern and pattern density of the embossed contact surfaces 256B, 258B.

In one illustrative embodiment shown in FIG. 2, the embossed contactsurfaces 256B, 258B comprise a plurality of grooves 231 that areconcentric with the substrate support 116. Each groove has a width 261and depth 259 of about 4 and 3 mm, respectively, and the grooves areseparated from one another by a wall having a thickness 257 of about 3mm.

In alternative embodiments, the embossed contact surfaces 256B, 258Beach may comprise a plurality of parallel grooves, orthogonal grooves,grooves separated by walls having different thicknesses, and the like.To reduce temperature non-uniformity across the substrate, theembossments generally have a higher pattern density in areas that opposehotter zones of the substrate support plate 160, the embedded heater132, or the substrate 114.

Thermally conductive sheets 213 may be placed between one or moresurfaces of the components comprising the substrate support 116. In oneembodiment, the thermally conductive sheets 213 are placed between thebottom surface 133 of the substrate support plate 160 and the embeddedheater 132 (shown in FIG. 2), the bottom surface 202 of the embeddedheater 132 and the top surface 256A of the first contact plate 256 (notshown), the embossed surface 256B of the first contact plate 256 and thetop surface 203 of the heat spreader plate 254 (not shown), the bottomsurface 204 of the heat spreader plate 254 and the top surface 258A ofthe second contact plate 258 (not shown), and the embossed surface 258Bof the second contact plate and the top surface 205 of the cooling plate166 (not shown). Each thermally conductive sheet 213 has cutouts thatconform to the surfaces of the components they separate to allow passageof lift pins 172 as well as the gas conduit 149. The thermallyconductive sheets 213 facilitate uniform heat transfer between thecomponents comprising the substrate support 116, when such componentsare compressed by bias members 190.

The thermally conductive sheets 213 may comprise graphite (GRAFOIL®flexible graphite commercially available from UCAR International, Inc.,Nashville, Tenn.), aluminum, and the like. The thickness of thethermally conductive sheets 213 should be within a range of about 1-5micrometers.

The heat transfer assembly 164 described herein may also be used toimprove the temperature uniformity of a substrate placed on a substratesupport plate (e.g., electrostatic chuck) having a detachable heater or,alternatively, a substrate support plate having an embedded heater.

To facilitate isolation of the interior region 186, the base plate 168is supplied with gas-tight seals 281, 283, and 285. Together, the sealsand collar ring 184 isolate the interior region 186 from the reactionvolume 141 (seal 281), guide hole 188 (seal 283), and gas conduit 149(seal 285). In the illustrative embodiment shown in FIG. 2, each suchseal comprises an elastic member (e.g., O-rings and the like) that isdisposed in a conventional manner in a circular groove.

Those skilled in the art will readily realize other permissiblemodifications of the substrate support 116 and heat transfer assembly164 that facilitate advantageous in-situ control of the substratetemperature and temperature non-uniformity.

While foregoing is directed to the illustrative embodiment of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

1. A heat transfer assembly for a substrate support in a substrateprocessing system, comprising: a first contact plate; a second contactplate disposed below the first contact plate; and a heat spreadingmember sandwiched between the first and the second contact plates,wherein the heat transfer assembly is configured to be disposed betweena heat source and a heat sink in a substrate support and to reducetemperature non-uniformity caused by features formed in the substratesupport, and wherein the first and second contact plates reduce heatflux through the heat transfer assembly.
 2. The heat transfer assemblyof claim 1, wherein the heat spreading member has a higher thermalconductivity than the first and second contact plates.
 3. The heattransfer assembly of claim 1, wherein the heat spreading member isbetween about 3-12 millimeters thick.
 4. The heat transfer assembly ofclaim 1, wherein the first and the second contact plates are betweenabout 3-12 millimeters thick.
 5. The heat transfer assembly of claim 1,wherein the first and second contact plates each comprise one smoothcontact surface and one embossed contact surface.
 6. The heat transferassembly of claim 5, wherein the embossed contact surface comprises anarea that is about 5 to 50% of the surface area of the smooth contactsurface.
 7. The heat transfer assembly of claim 5, wherein embossment ina first region of the embossed contact surface differs from embossmentin at least one other region of the embossed contact surface.
 8. Theheat transfer assembly of claim 5, wherein the embossed contact surfaceof the first contact plate contacts an upper surface of the heatspreader member and a the smooth contact surface of the second contactplate contacts a bottom surface of the heat spreader member.
 9. The heattransfer assembly of claim 1, wherein the heat spreading member isformed of a material selected from the group consisting of aluminumnitride (AlN) and copper (Cu).
 10. The heat transfer assembly of claim1, wherein the first and second contact plates are formed of a materialselected from the group consisting of titanium (Ti) and an alloycomprising iron (Fe), nickel (Ni) and cobalt (Co).
 11. The heat transferassembly of claim 1, wherein a thermally conductive sheet is disposed inleast one of the following locations: adjacent a top surface of thefirst contact plate, immediately between the first contact plate and theheat spreader member, immediately between the heat spreader member andthe second contact plate, or adjacent a lower surface of the secondcontact plate.
 12. The heat transfer assembly of claim 11, wherein thethermally conductive sheet is formed of a material selected from thegroup consisting of graphite and aluminum.
 13. The heat transferassembly of claim 11, wherein the thermally conductive sheet is betweenabout 1-5 micrometers thick.
 14. Apparatus for processing asemiconductor substrate, comprising: a process chamber; and a substratesupport disposed in the process chamber comprising a heat spreadingmember sandwiched between a first contact plate and a second contactplate, wherein the first contact plate is thermally coupled to a heatsource and the second contact plate is thermally coupled to a heat sink,wherein the heat spreading member is configured to reduce temperaturenon-uniformity caused by features formed in the substrate support, andwherein the first and second contact plates reduce heat flux from theheat source to the heat sink.
 15. The apparatus of claim 14, wherein theheat source is an embedded heater.
 16. The apparatus of claim 15,wherein the embedded heater is a detachable heater that is thermallycoupled to a substrate support plate of the substrate support.
 17. Theapparatus of claim 14, wherein the first and second contact plates eachcomprise one smooth contact surface and one embossed contact surface,wherein the embossed contact surface of the first contact plate contactsan upper surface of the heat spreader member and a the smooth contactsurface of the second contact plate contacts a bottom surface of theheat spreader member.
 18. The apparatus of claim 17, wherein embossmentin a first region of the embossed contact surface differs fromembossment in at least one other region of the embossed contact surface.19. The apparatus of claim 14, wherein the heat spreading member isformed of a material selected from the group consisting of aluminumnitride (AlN) and copper (Cu).
 20. The apparatus of claim 14, whereinthe first and second contact plates are formed of a material selectedfrom the group consisting of titanium (Ti) and an alloy comprising iron(Fe), nickel (Ni) and cobalt (Co).
 21. The apparatus of claim 14,further comprising at least one bias member that engages and improvesthermal coupling between a substrate support plate, the heat source, theheat transfer assembly, and the heat sink.
 22. The apparatus of claim21, wherein each bias member exerts an expanding elastic force and isdisposed in the heat sink or a base member coupled to the heat sink. 23.The apparatus of claim 14, wherein a thermally conductive sheet isdisposed immediately between at least one of the following: a bottomsurface of a substrate support plate and the heat source, a bottomsurface of the heat source and a top surface of the first contact plate,a bottom surface of the first contact plate and a top surface of theheat spreader member, a bottom surface of the heat spreader member and atop surface of the second contact plate, or a bottom surface of thesecond contact plate and a top surface of a heat sink.
 24. The apparatusof claim 23, wherein the thermally conductive sheet is formed of amaterial selected from the group consisting of graphite and aluminum andis between about 1-5 micrometers thick.
 25. A substrate support,comprising: a heat source disposed beneath a support surface of thesubstrate support; a heat sink disposed beneath the heat source; and aheat transfer assembly thermally coupling the heat source to the heatsink, the heat transfer assembly comprising: a heat spreading membersandwiched between a first contact plate and a second contact plate,wherein the first and second contact plates reduce heat flux through theheat transfer assembly.